This invention relates to the measurement of the thermal behavior of materials, and, more particularly, to a method and apparatus which uses measurement of the thermal behavior for determination of the microgeometry of porous media such as rocks. An important form of the invention is the logging of earth formations surrounding a borehole.
The determination of the microgeometrical structure of porous media, such as rocks and other naturally occurring materials, poses various difficulties, including the fact that many different length scales are typically involved. Neutron diffraction can probe a material from atomic dimensions up to several Angstroms. For much larger structure, on the order of 1 mm, magnetic resonance can be used. However, for the intermediate length scales, there is no satisfactory method for non-destructive determination of pore geometry information. This limitation is particularly noteworthy in that it is these length scales, around 1 micrometer, which are important for determination of physical properties such as fluid flow through a material in question.
The difficulties associated with non-destructive measurement of the pore characteristics of media are, of course, exacerbated when one needs to measure materials in an in situ environment like an earth borehole. The determination of permeability and other parameters which depend upon the microgeometry of formations, is very useful in gauging the hydrocarbon producibility of the formations. However, existing techniques for measuring permeability and related parameters in a borehole have various drawbacks. So-called "direct" measurement techniques involve measurement of fluid flow, pressure, etc. in the formations, and require logging tools which inject and/or extract fluids to and/or from the formations. This technique generally involves measurement devices with moving parts which have limited reliability, operating speed, and data acquisition capabilities. In so-called "indirect" methods, permeability is determined from empirical correlations which attempt to express permeability in terms of other measured formation parameters, for example porosity and saturation. These techniques can have limited accuracy in various situations.
It is among the objects of the present invention to provide a method and apparatus for determining microgeometrical properties of materials using non-destructive investigation. While this objective extends to measurement of materials in any environment, it is a particularly important object to devise such a technique which can be employed in well logging applications for investigation of earth formations surrounding a borehole.